Generally, by changing the ratio between copper and nickel, copper-nickel alloys are made to exhibit excellent properties in corrosion resistance, malleability/ductility, processability, and high temperature characteristics, and copper-nickel alloys also have characteristic properties in electric resistivity, coefficient of thermal resistance, thermal electromotive force, coefficient of thermal expansion, and the like. Thus, studies have hitherto been conducted to obtain such properties of copper-nickel alloys by electroplating. As conventionally attempted copper-nickel alloy electroplating baths, a large variety of baths have been studied, including a cyanide bath, a citric acid bath, an acetic acid bath, a tartaric acid bath, a thiosulfuric acid bath, an ammonia bath, a pyrophosphoric acid bath, and the like; however, none of these baths have been put into practical use.
The reasons why the copper-nickel alloy electroplating has not practically been used are as follows:
(1) copper and nickel differ from each other in deposition potential by approximately 0.6 V, so that copper is preferentially deposited;
(2) the plating bath is so unstable that insoluble compounds such as metal hydroxides are formed;
(3) the plating composition varies due to energization, so that a coating having a uniform composition cannot be stably obtained;
(4) the service life of the liquid is short; and the like.